class Solution {
public:
    int pivotIndex(vector<int>& nums) {
        int n = nums.size();
        vector<int> pre(n, 0);
        vector<int> suf(n, 0);
        for (int i = 1; i < n; i++)
        {
            pre[i] = pre[i - 1] + nums[i - 1];
        }
        for (int i = n - 2; i >= 0; i--)
        {
            suf[i] = suf[i + 1] + nums[i + 1];
        }
        for (int i = 0; i < n; i++)
        {
            if (pre[i] == suf[i])
                return i;
        }
        return -1;
    }
};